This invention relates to a device for evaluating authenticity features based on the diffraction patterns produced by irradiating diffraction elements with optical energy.
Many different techniques are used to authenticate documents and things which are subject to counterfeiting or forgery. By way of example only, documents which are particularly in need of authentication include bank notes, identification papers, passports, packagings, labels and stickers, drivers licenses, admission tickets and other tickets, tax stamps, pawn stamps and stock certificates. Typically, an xe2x80x9cauthentication elementxe2x80x9d is printed or otherwise attached to the document or thing which is to be secured. The authentication element can be analyzed when it is necessary to authenticate the document.
It is known to use diffraction elements for authentication purposes. Such diffraction elements are usually holograms. One type of hologram contains a diffraction grating on the surface of the element. The diffraction grating is formed by elevations and impressions made by embossing. Another of type of hologram (known as a xe2x80x9cthick hologramxe2x80x9d) contains a diffraction grating within the volume of the material. The diffraction grating is formed by local differences in optical density caused by laser treatment. In the preferred embodiment, a diffraction element may comprise either of the foregoing elements but the invention is not limited to a particular type of diffraction element and concerns instead the apparatus for evaluating the element to establish authenticity of a secured document.
PCT application No. WO 98/55963 discloses the use of a diffusion element or marking on a document which, when irradiated by a laser, will diffract and reflect the laser beam in the form of a characteristic pattern. The diffraction pattern produced is projected onto photodiodes which are fixed in position within a housing at sites where the characteristic diffraction pattern is expected. Authenticity of the document can then be determined based on whether or not the photodiodes are receiving a diffraction pattern. For a system to work, the photodiodes must be disposed at predetermined fixed positions. If the diffraction element or marking is displaced or changed slightly on the surface of a document, or if a different type of diffraction element is used, the diffraction pattern will not be received at the fixed photodiodes. Hence, this particular device can be used only with documents having the same diffraction elements in the same place on the document.
PCT application WO 98/55963 discloses that the photodiodes may be arranged in the form of a matrix in order to receive a larger range of diffraction patterns. Nevertheless, it is not practical to use photodiodes which are fixed in position to determine different diffraction patterns at different places where intensity differs.
A further disadvantage of photodiodes is their coarse resolution, which is far less than the high-resolution capability of matrix cameras or cell cameras with light-sensitive CCD or CMOS sensors. They are therefore not suitable for smaller, finely distributed distinguishing authenticity features.
It is therefore an object of the invention to provide a device of the type described above in which a greater diversity of distinguishing authenticity features based on diffraction elements can be identified, and the checking of the authenticity of a document is independent of the position at which the distinguishing authenticity feature(s) containing the diffraction element(s) are fixed on the document.
In accordance with the invention, the diffraction patterns derived from the document to be examined are imaged onto a projection surface and the diffraction picture, formed on the projection surface, is observed and evaluated by a camera which covers the projection surface.
The advantage of the invention is that the places at which the diffraction patterns are incident on an examining surface are no longer fixed. It is therefore immaterial where the diffraction pattern is incident. The whole of the projection surface is observed with a camera (matrix or cell camera) and the characteristic diffraction image formed on the surface is evaluated. The evaluation of this diffraction image is a measure of the authenticity of the document.
With the invention, light-sensitive sensors no longer have to be disposed at fixed places on a projection surface; instead, different, even variable, diffraction patterns can be evaluated, because the projection of the diffraction pattern is examined by camera on a projection surface.
Different diffraction structures of different natures and at different places on the document can be examined simultaneously or consecutively, because only the diffraction pattern cast by the diffraction element(s) onto the projection surface is examined by camera.
The expression xe2x80x9cprojection surfacexe2x80x9d is generally understood. In one embodiment, a matt disk onto which the diffraction patterns are reflected, can be provided as a projection surface. A camera, which evaluates the diffraction pattern on the matt disk, is disposed below the disk.
The camera may image and evaluate regions of the matt disk or the entire surface of the matt disk in a single step.
If the camera is separated from the matt disk, it is immaterial in accordance with the invention whether the camera moves with the optionally movable matt disk, or the matt disk alone is moved and the camera is stationary.
Moreover, for evaluating a diffraction element in a document, there are several different embodiments, all of which are included in the invention.
In a first embodiment, the laser provided for illuminating the diffraction element in the document, is constructed so that it can move only, for example, in the X direction and is stationary in the Y direction.
In a different embodiment, the laser may be fixed immovably, for example, to the housing and cannot be deflected itself, but the beams of the laser are directed onto a rotating mirror, which can be deflected over a certain angular range. The laser beam which is deflected by the rotating mirror is then directed over a telecentric mirror, which is fixed in a position in the housing, onto the diffraction element in or on the document.
In both embodiments, the matt disk may be movable in the Y direction (scanning direction) with a stationary deflection mirror disposed on the matt disk so that the beam generated by the laser is passed over the mirror onto the stationary diffraction element in the document.
In a first embodiment, the deflection mirror for the laser beam takes up approximately the whole width of the matt disk. This arrangement has the advantage that the matt disk itself does not have to be moved in the X direction.
In a different embodiment, the width of the deflection mirror is significantly less than the width of the matt disk, and the entire matt disk is movable in the X as well as the Y direction so that it can be moved under the stationary diffraction element, the laser then being coupled to the matt disk.